1. Field of Application
The present invention relates to a control apparatus for controlling a control quantity of a polyphase rotary machine, by controlling an inverter (electric power converter circuit) having switching elements connected to the phases of the rotary machine, the switching elements being operated to selectively connect each of the phases to at least two different DC voltage levels.
2. Description of Related Art
It is known, for example as described in Japanese patent application publication No. 2006-174697 (designated in the following as reference 1), that such a type of control apparatus can utilize predictive control by applying a mathematical model, for maintaining the torque or stator magnet flux values of a rotary machine within a predetermined range while restricting the switching frequency of the switching elements. With the control method of reference 1, at each of successive periodic timings (k−1), k, (k+1), . . . , an appropriate operation state of the inverter (combination of on/off condition of the switching elements) is determined, to be established at the next timing. With the described embodiment, the inverter can apply three different voltage levels (+v, 0, −v) to each of the 3 phases of the rotary machine, i.e., there is a total of 27 (33) operation states of the inverter. As illustrated in the state diagram of FIG. 3 of reference 1, a plurality of sequences of operation states can extend from each operation state, by respective paths. With a specific operation state having been precedingly established for the current timing (k), all of the possible operation state sequences that extend from that timing (k) are first determined, up to a predetermined number N of future timings, i.e., over a time range k, (k+1), . . . (k+N−1) (where N≧1). Considering only control of torque, for each of the future operation states corresponding to these future updating timings, a mathematical model of the rotary machine is applied to calculate corresponding predicted values of torque developed by the rotary machine. Each locus of such predicted values is referred to as a trajectory, i.e., a corresponding trajectory of predicted torque values is obtained for each of the possible operation state sequences that extend from the present timing.
FIG. 13 of the drawings of the present application illustrate this, for the case of two possible torque trajectories T1 and T2 extending from the timing k, with a trajectory T3 branching from the trajectory T1 at the timing (k+1). Prior to the timing (k+1), the operation state to be established at that timing (k+1) is determined as follows. The time range of the prediction calculations extends to the timing (k+2) in this example. As shown, the trajectory T3 is predicted to depart from a predetermined range of torque values before the timing (k+2) is reached. When the trajectory T2 is extrapolated, it is predicted to depart from the predetermined torque range at approximately the timing (k+3), while the T1 (extrapolated) is predicted to remain within the predetermined torque range for a longer time.
Each of these trajectories is then examined to find the total number of successive operation state transitions until the trajectory is predicted to depart from the predetermined torque range (for example, one transition in the case of the trajectory T3, two transitions for trajectory T2). The total number of switching element (on or off) operations required to effect these transitions is then calculated, for each of the trajectories.
An evaluation coefficient is then calculated, for each trajectory, by dividing the corresponding total number of switching element operations by the corresponding total number of operation state transitions. The trajectory having the smallest value of evaluation coefficient (i.e., the lowest frequency of switching element changeovers) is then selected as optimum. Hence, the operation state which is next in the operation state sequence of that optimum trajectory is selected, to be set as the next operation state. For example if the trajectory T2 in the above example were judged (at timing k) to be optimum, then the operation state indicated as OS1 would be set as the actual operation state of the inverter at timing (k+1).
The above processing is repeated at each of successive time steps. The switching frequency of the inverter can thereby be limited.
Another example of a related prior art control method, having similar objectives, is described in “A Study on Current Control System of PMSM Operating at High Speed Based on Model Predictive Control” by Kadota et al, IEE All-Japan Conference 2006, volume 4, pp. 175-176. The method is described for application to a PMSM (permanent magnet synchronous motor) driven by an inverter. For each of successive intervals referred to as slots, model prediction is applied to the operating parameters of the PMSM for determining a pattern (sequence of switching modes of the inverter) which will result in a minimum number of changeovers of the switching elements during that slot, while ensuring that the drive current of the motor will be in accordance with a command value.
However with such prior art methods, although the switching frequency of the inverter can be limited, it is not possible to avoid a condition whereby the voltage levels applied to a plurality of the phases of the rotary machine are switched concurrently. When such a condition occurs, voltage surges appear across the input/output terminals of the switching elements and across a smoothing capacitor connected between the input terminals of the inverter. As a result, the withstanding voltage requirements for the switching elements and the smoothing capacitor are increased accordingly. This results in increased cost and increased size of the hardware of the inverter.